Re: [multipathtcp] draft terminology: MPTCP control block variable SND.NXT

Hi Nigel,

On Monday 12 November 2012 17:53:37 Nigel Williams wrote:
>  From the draft [1] v12, p59, Section B.1.2 (variables in the MPTCP
> control block):
> "SND.NXT (64 bits):  This is the Data Sequence Number of the next byte
> to be sent.  SND.NXT is used to determine the value of the DSN in the
> DSS option."
> 
> This terminology is borrowed from regular single flow TCP, but it's
> meaning does not seem to us to be the same in the MPTCP context. When a
> subflow asks the scheduler for new data to send, the scheduler allocates
> a chunk of the TX socket buffer to the subflow, tells the subflow the
> starting DS value for the chunk and the length of the chunk. The subflow
> then proceeds to send a DS map for the chunk and all the associated data
> before calling back up to the scheduler to get a new chunk allocated.
> 
> Each subflow's internal concept of snd.nxt makes sense as in the regular
> single flow TCP case, but at the DS level, packets leave the machine
> with DS-level sequence numbers that jump all over the place (when using
> mappings > 1-MSS on multiple subflows).

This depends on the scheduler, and I don't see why mappings > 1-MSS affects 
this.

Is the data leaving the sender out-of-order on the data-sequence number space 
in your implementation?

In our implementation, even if the mapping would be > 1-MSS, the SND.NXT is 
simply increasing as new data is pushed out on the network.
I believe that. sending the data in-order on the DS-level allows an easier 
handling of the meta-level retransmission timer.

Cheers,
Christoph

> The interesting variable at the
> DS level is really the largest DS value mapped to a subflow i.e. the
> right most edge of the TX socket buffer which designates the boundary
> between bytes which have been mapped into subflows and bytes which have
> not. This is not analogous to an individual TCP flow's SND.NXT though,
> hence the point of discussion.
> 
> cheers,
> nigel
> 
> 
> [1] http://www.ietf.org/id/draft-ietf-mptcp-multiaddressed-12.txt
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MultiPath TCP in the Linux Kernel --- http://mptcp.info.ucl.ac.be
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